A London teenager was jailed for life on Friday for plotting a terror attack, becoming the youngest woman to be sentenced for terrorism offences in Britain.

Safaa Boular, 18, who together with her mother and elder sister formed the country's first all-female cell linked to the Islamic State group, will spend a minimum of 13 years in prison.

She had planned violent attacks in the British capital, using coded language themed around a tea party, after UK authorities stopped her travelling to Syria to marry an IS fighter she had met online.

"However much she may have been influenced and drawn into her extremism, it appeared she knew what she was doing and acted with open eyes," said Judge Mark Dennis in sentencing her at London's Old Bailey.

"Her views were deeply entrenched," he added, rejecting claims she had renounced extremism.

In June sibling Rizlaine Boular, 22, was also jailed for life, with a minimum term of 16 years, while their Morocco-born mother Mina Dich was handed a minimum sentence of six years and nine months for aiding the plots.

Safaa Boular was only 16 when she made contact with British-born IS fighter Naweed Hussain, 32, discussing marriage and how they would don his-and-hers suicide belts.

But her hopes of joining him were dashed when she was stopped at the airport in August 2016 following a family trip to Morocco, and her passport was confiscated.

'Family with murderous intent'

Instead Boular decided to plan an attack in Britain, detailing it in coded language -- such as using "pineapples" for grenades -- to online contacts, who were in fact undercover agents.

Hussain was later killed in a drone strike.

When Boular was charged with preparing terrorist acts in April 2017, she handed responsibility to her sister Rizlaine, 22, and their mother Mina Dich, 44, who hatched their own plan.

The trio were taped talking about an Alice in Wonderland-themed tea party, which the prosecution argued was code for an attack.

Rizlaine Boular and Dich were arrested after being tracked by police visiting potential sites around Westminster and buying knives.

They pleaded guilty to terror offences, but Safaa Boular denied involvement in the plot.

She remained expressionless as she was jailed Friday for two counts of preparing terrorist acts after being found guilty at trial in June.

Dean Haydon, Britain's top counter-terror police officer, said then the plot "involved a family with murderous intent, the first all-female terrorist plot in the UK connected to Daesh (IS)".

He added: "All three women were filled with hate and toxic ideology and were determined to carry out a terrorist attack.

"Had they been successful, it could well have resulted in people being killed or seriously injured."